In motor vehicle multispeed ratio automatic transmissions, it is often desirable to shift from one speed ratio to another without the use of freewheeling or one-way devices. This requires a coordinated timing control of both off-going and on-coming fluid operated torque transmitting devices in order to achieve a desired amount of overlap as the transmitted torque is shifted from the off-going device to the on-coming device. Typically, the pressure supplied to the off-going device is progressively released through an orifice, while fluid pressure is supplied to the on-coming device through an accumulator or a servo in which an output element is displaced (stroked) by the supplied pressure.
Where a control of the on-coming device engagement time is desired, the fluid flow at the inlet or outlet of the accumulator or servo may be separately regulated. In one known system involving a servo actuated friction band device, for example, a speed-biased regulator valve is used to vary a restriction for fluid being displaced by the servo piston. In another similar system, an electrohydraulic valve is pulse-width-modulated at a variable duty cycle to vary a restriction at the fluid inlet of the servo. Unfortunately, these systems are relatively expensive to implement and often exhibit some level of supply pressure sensitivity, degrading the ability of the control to achieve the desired on-coming engagement time.